The present invention relates to a zoom lens system for a still camera. More particularly, the present invention relates to a zoom lens system that attains a zoom ratio of about 2 by covering a half view angle ranging from about 32.degree. at the wide angle end to about 17.degree. at the semi-telephoto end and which is very compact despite its capability for attaining a brightness (i.e., large effective aperture) at F.sub.NO of 1:2.8 or in the range of F.sub.NO 1:2.8 to 1:3.5.
Zoom lens systems that attain a zoom ratio of about 2 by covering half view angles ranging from wide angle to semi-telephoto and which feature aperture ratios in the range of 1:3.5 to 1:4.5 are classified either as a two-group type which consists of a negative first lens group and a positive second lens group as disclosed in Japanese Patent Application (OPI) Nos. 64811/1984 and 142515/1984 (the term OPI as used herein means an unexamined published Japanese patent application) or as a third-group fixed type which includes a fixed negative third lens group additionally disposed behind the combination of the negative first lens group and the positive second lens group as shown in Japanese OPI Nos. 97016/1983, 111013/1983 and 240217/1986. Zoom lens systems that feature an aperture ratio on the order of 1:2.8 while attaining a zoom ratio of about 2 are shown in Japanese OPI Nos. 83543/1986 and 9550/1980 and they all relate to the two-group type.
Problems with the prior art zoom lens system featuring F.sub.NO of 1:3.5 to 1:4.5 will now be described. Japanese OPI No. 64811/1984 shows that the overall length of a lens system can be reduced by providing an aspherical surface in a first lens group which is composed of two lens elements. However, the power of the first lens group is so small that it has to be displaced by a large amount during zooming and the overall length of the lens system is by no means short at the wide-angle end. In addition, the focusing lens has to be advanced by too large an amount to make the closest focusing distance short enough.
The zoom lens system shown in Japanese OPI No. 142515/1984 has a larger overall length than the system disclosed in Japanese OPI No. 64811/1984 because of the absence of any aspherical lens surface. Generally speaking, a zoom lens system of the two-group type does not allow the second lens group having a diaphragm stop to be moved by a smaller amount than in a lens system of the third-group fixed type having the same overall length (as will be discussed later in this specification).
The zoom lens system of the third-group fixed type which is a rather recent version is essentially the same as the zoom lens system of the two-group type except that it additionally includes a rear converter. The system shown in Japanese OPI No. 97016/1983 has the advantage of experiencing a small distortion. On the other hand, probably because of its intended application as a lens system for medium-format (6.times.4.5 cm or 6.times.7 cm) camera, this lens system has the disadvantage that it has a large overall length and requires many lens elements (especially in the first and third lens groups). The system shown in Japanese OPI No. 111013/1983 has the advantage of small size but, on the other hand, the negative powers of the first and third lens groups are so great that substantial variations will occur in spherical aberration and astigmatism during zooming. Furthermore, the excessive power of the first lens group causes the disadvantage that the second lens group has to be displaced by a large amount in spite of the use of the third lens group.
A zoom lens system that employs a negative third lens group which is not fixed but movable toward the object side together with the second lens group is proposed in many prior applications such as Japanese OPI Nos. 26754/1979 and 59157/1979. However, as compared with the third-group fixed type, this system has the disadvantage that the second lens group having a diaphragm stop has to be displaced by a large amount and that the overall length of the system is by no means small.
With a view to solving these problems, the assignee of the present invention filed Japanese OPI No. 240217/1986 and proposed a zoom lens system having a shorter overall length than any of the prior art products. However, there still remains room for improvement of this system because its large F number is not well-suited to an obvious requirement for a bright and compact system.
Problems with the prior art lens systems featuring F.sub.NO on the order of 1:2.8 will now be discussed.
The zoom lens system shown in Japanese OPI No. 83543/1986 is very simple for an aperture ratio of 1:2.8, since it is composed of 7 elements in 7 units. However, it suffers the disadvantage of large overall length. The system proposed in Japanese OPI No. 9550/1980 is also unsatisfactory in that it is capable of attaining a zoom ratio of less than 1.6 and that its overall length is by no means small. Each of these systems is of the two-group type and no system has been available in the art that is of the third-group fixed type and which features an aperture ratio on the order of 1:2.8. This is probably because, with a lens system as bright as one having an F.sub.NO of 1:2.8, effective aberrational compensation cannot be attained if comatic flare is increased by using a third lens group having a magnification greater than unity.